In a plasma arc torch, a high voltage is supplied to the electrode to create an electrical arc extending from the electrode and through the bore of a nozzle assembly. A flow of gas is generated between the electrode and the nozzle assembly to create a plasma flow through the bore to a workpiece positioned beneath the nozzle assembly. The high heat and electrical arc often damage the consumable components of the torch, such as the nozzle assembly and the electrode, and as a result, these components must be replaced. Typically, these components are threaded onto the torch body, and an operator unscrews the components from the torch body and replaces them when necessary.
During replacement of the consumable torch components, a danger exists that an operator may receive an electric shock if voltage still is supplied to the torch electrode. Without the protection afforded by the plasma torch's heat shield and nozzle assembly, the electrode is exposed. An operator may accidentally touch the electrode, causing an electric shock.
Some plasma arc torch designs include safety mechanisms which prevent electrical voltage to the electrode when the nozzle assembly or heat shield are removed or partially disassembled from the torch body. For example, U.S. Pat. Nos. 4,701,590, 4,959,520, and 4,973,816 disclose plasma arc torch designs in which spring actuated pistons and other parts move or slide within the torch body after the heat shield or nozzle assembly is removed.
These moving parts actuate a control mechanism which prevents electrical voltage to the electrode, thus preventing electrical shock of the operator who replaces the torch consumables. Although these torch safety mechanisms provide some measure of safety, moving pistons or other moving parts are not preferred because the torch not only is more complex and expensive with such safety systems, but also the torch life may be lessened without the additional maintenance necessary with the more complex safety systems having moving parts.
Other plasma arc torch safety systems propose a plasma arc torch interlock in which a nonmovable fault detect circuit senses a short between the electrode and the nozzle and disables the electrode power supply when the short is sensed. In U.S. Pat. No. 4,929,811, the fault detector circuit comprises a main cable which when punctured, contacts a main conductor, and in response to that contact, actuates a fault detector circuit to disable the power source that generates voltage to the electrode. In another embodiment, a spring wire provides continuity contact with a nozzle assembly to complete a closed loop circuit, which when broken, disables the electrode power source. However, in both embodiments, the heat shield can be removed without disabling the torch, resulting in a still dangerous situation in which the operator performing maintenance or replacement of the torch could receive an electrical shock.